Enterasys Networks Security Router X-PeditionTM - Manual (Page 292)

Mechanisms Providing QoS
12-10 Configuring Quality of Service
queue-limit value for the queue size. Be aware that by setting the queue size smaller than the
shaper burst, shape will not be able to achieve the configured average rate. When the queue-limit
command is not invoked, queue size is determined only by the shaper burst.
Congestion Control & Avoidance
Describing Queue Size Control (Drop Tail)
By using delay control and congestion avoidance, you control queued up packets. If an outgoing
queue is empty when a packet is ready to be sent, the packet can be forwarded immediately to the
line with minimal delay. But, if there are 20 queued packets in the outgoing queue when the
packet arrives, the new packet must wait until the 20 queued packets are sent before it can go.
Depending on the average packet size of the queued packets and the speed of the link, this last
packet could be delayed considerably. When the queue limit is reached no new arriving packets
are accepted in the queue and are dropped. The limit of the queue is set by the
queue-limit
command as shown in the following example:
XSR(config)#policy-map droptail
XSR(config-pmap<droptail>)#class the_heat
XSR(config-pmap-c<the_heat>)#queue-limit 50
Describing Random Early Detection
Random Early Detection (RED) is a congestion avoidance mechanism for adaptive applications
(e.g., TCP/IP) that adjusts bandwidth usage of the XSR based on network conditions. TCP/IP uses
a slow-start feature that initially sends a few packets to test network conditions.
If the acknowledgement returns indicating no packet loss, TCP considers the network capable of
handling more traffic and increases its output rate. The protocol continues to do so until it detects
any packets dropped and not delivered, at which point it considers the network congested and
begins cutting back the output rate.
Because of TCP’s slow-start/fast-drop-off behavior when dealing with congestion, TCP/IP’s
performance is choppy when the node or network is heavily loaded and the network does not
apply congestion avoidance. This occurs because when the node is congested and the outgoing
queue fills up, subsequent packets (very likely from multiple TCP sessions) are dropped, and
these in turn cause corresponding TCP sessions to dramatically cut output.
After a short delay, all sessions try to ramp up using slow-start in a process called Global
Synchronization. The queue grows, congestion and packet drops recur, and undesirable global
synchronization repeats. The end result is a distinctive “peak and trough” traffic pattern where
the outgoing queue is full just before packets are dropped, delay throughout the network is high
and varies by large margins.
RED tries avoiding congestion by proactively dropping packets randomly at an early sign of
congestion (when the queue rises above the threshold). Because packets are dropped randomly,
all TCP/IP sessions will be affected eventually and the treatment made fair to all sessions.
By dropping packets early - before it reaches its queue limit - RED starts to “throttle” the traffic
source before the queue grows too large. It helps limit delay, which is proportional to the number
of packets in the queue, and avoid queue overflow and global TCP synchronization.
The random-detect command includes three parameters to configure RED for a queue:
minimum threshold (MinThres), maximum threshold (MaxThres) and maximum drop probability
(MaxProb). The drop probability of a packet is based on the average queue size and the three
parameters mentioned earlier. The calculation of the drop probability is pictured below.